The present invention relates generally to generation of vacuum ultraviolet radiation, and more particularly to resonantly enhanced generation of wavelength tunable, coherent vacuum ultraviolet radiation utilizing thirdharmonic generation in gaseous carbon monoxide.
Interest in relatively intense, coherent, tunable light sources in the vacuum ultraviolet region of the electromagnetic spectrum has increased in recent years because of the growth of spectroscopic investigations in this wavelength region; e.g., using synchrotron radiation sources, and the need for a diagnostic procedure for hydrogen in fusion research, to name two reasons. The method of the instant invention provides wavelength tunable coherent radiation in the region of the ultraviolet between 154.4 and 124.6 nm by means of third-harmonic generation in carbon monoxide resonantly enhanced by the nearby presence of the A.sup.1 .pi. state. That is intense, pulsed visible radiation in the range 463.2 to 373.8 nm is generated by a tunable dye laser and focused into a gaseous sample of carbon monoxide, and due to the proximity of a series of vibronic energy levels the third order nonlinear susceptibility of carbon monoxide is rendered large, thereby allowing strong four-wave sum mixing to occur with the emission of coherent vacuum ultraviolet radiation at one-third the wavelength of the incident visible or near ultraviolet radiation. By making use of the multitude of vibration-rotation states within the vibronic manifold, by varying the carbon monoxide pressure, and by using isotopic carbon monoxide .sup.13 CO or C.sup.18 O the range from 154.4 to 124.6 nm can be nearly continuously covered. The major problem when evaluating multiple-photon harmonic generation schemes in materials is that very high pump laser intensities are generally necessary to achieve significant harmonic output. Such substantial laser electric fields are sufficient to dissociate or ionize the nonlinear medium. However, if a real molecular energy level exists which is closely matched to some multiple of the pump laser frequency (in the instant case, a factor of 3) a resonant enhancement of the third-order nonlinear susceptibility occurs which significantly increases the magnitude of this quantity so that the experimentally observed third-harmonic signal is increased by many orders of magnitude. That is, the intensity of the generated third-harmonic signal is proportional to the square of the magnitude of the third-order nonlinear susceptibility, so if this quantity is made large because of the near coincidence of a nearby molecular energy level with which the pump photons can interact, the coherent, tunable generated radiation can be made quite intense. The purpose of this resonant enhancement is not to induce actual transitions (although these will occur to a small extent), but to maximize the interaction between the pump laser photons and the nonlinear medium, here carbon monoxide. In other words, there is a nonlinear interaction between the pump, radiation and the carbon monoxide without significant absorption, which is a linear process. This concept will be an important distinguishing feature of the instant invention over the first of the five relevant references discussed below.
1. "Three-Photon Excitation of Xenon and Carbon Monoxide," by F. H. M. Faisal, R. Wallenstein, and H. Zacharias, Phys. Rev. Letters, 39, 1138 (1977), describes the excitation through the P, O, and R branches of the (2-0) band of CO in the fourth positive system (A.sup.1 .pi..rarw.X.sup.1 .SIGMA..sup.30 ). This A.sup.1 .pi. state is the energy level which is used in the instant invention to resonantly enhance the third-harmonic generation. However, the authors report only the observation of fluorescence orthogonal to the pump laser beam. In addition, they find that this fluorescence disappears as the CO pressure is increased. These two facts strongly teach away from our method. First, the coherent third-harmonic radiation taught by the instant invention is not isotropic; that is, it is not radiated in all directions so that it would not be detected orthogonally to the pump laser beam. Moreover, the fluorescence radiation consists of much spectroscopic detail whereas, our coherent radiation is simply a single, narrow feature. Finally, the disappearance of the fluorescence with pressure is most likely due to the increased efficiency of third-harmonic generation which competes with the actual level pumping by three-photon absorption. At increased pressure, the conditions required for harmonic generation are improved due to a higher nonlinear susceptibility of the CO medium.
2. "Four-Wave Sum Mixing (130-180 nm) in Molecular Vapors," by K. K. Innes, B. P. Stoicheff, and Stephen C. Wallace, Appl. Phys. Letters, 29, 715 (1976), discusses the generation of coherent radiation in gaseous samples of nitric oxide, bromine and benzene by means of resonantly enhanced four-wave mixing. The authors thereon merely mention that bromine provides 177 nm radiation, while benzene provides radiation at 163 nm. Nothing was related concerning the energy level schemes involved except that these gases yield lower harmonic conversion efficiencies than NO which was discussed in much greater detail. This reference is appropriately combined with reference 3 to be elaborated on hereinbelow and wherein two of the three authors investigate harmonic generation in nitric oxide in yet further detail.
3. "Nonlinear Laser Spectroscopy in Nitric Oxide Studied Through VUV Harmonic Generation," by Stephen C. Wallace and K. K. Innes, J. Chem. Phys., 72, 4805 (1980) shows the energy level scheme used for resonantly enhancing third-harmonic generation for nitric oxide. FIG. 1 therein shows that the enhancing level is coincident with the energy reached by two photons, there being no molecular energy level nearby the energy attained by three photons. This article teaches away from the instant invention in three respects. First, again referring to FIG. 1, the only optical mixing scheme proposed which utilizes the presence of an energy level of NO with energy corresponding to 150 to 130 nm (M.sup.2 .SIGMA..sup.+ incorrectly labeled as M.sup.2 .pi.), teaches the use of an intense first frequency of visible radiation such that there is a nearby energy level at the sum frequency for two of these photons, and a second visible frequency photon to span the energy gap between the sum of the two first resonant photons and a second energy level corresponding to vacuum ultraviolet radiation. An intense vuv four-wave mixing signal is observed. Basically, this is a double resonance mixing scheme which gives about 1000 times more output energy than the third-harmonic generation utilizing the single energy level at twice the pump photon energy and a third photon of the same energy to reach the vacuum ultraviolet region of the electromagnetic spectrum, which radiation is detected as a coherent signal. Further, in the conclusions section, the authors mention other molecules for which multiphoton spectra have been obtained (Br.sub.2, IBr, H.sub.2 O and benzene), but neither give details, nor mention the use of carbon monoxide. Moreover, in this same section, mention is made of the observation of third-harmonic generation in NO without intermediate resonant enhancement. The only other mention of third-harmonic generation was that mentioned hereinabove and utilizes the intermediate level in NO at twice the laser pump frequency. This is a very similar approach to that taken in Ref. 4 discussed hereinbelow. If it were obvious to make use of the clearly available energy level in NO corresponding to vuv emission to significantly enhance this emission, the authors of Refs. 2 and 3 have found three distinct methods of teaching away from the instant invention which teaches the use of such a level, thereby improving the intensity of the emitted radiation orders of magnitude over the methods taught in their above-referenced articles. It is reasonable to expect that at the higher laser intensity required for third-harmonic generation, even resonantly enhanced by a nearby energy level, that the NO would undergo significant destruction through ionization or dissociation in view of its instability relative to CO which was chosen for use in the instant invention because of its great stability and for its appropriate energy level system.
4. In "Third-Harmonic Generation Using An Ultrahigh-Spectral Brightness ArF* Source," by H. Pummer, T. Srinivasan, H. Egger, K. Boyer, T. S. Luk, and C. K. Rhodes, Optics Letters 7, 93 (1982), the authors teach the generation of tunable coherent radiation in the vicinity of 64 nm using third-harmonic generation of the output from an ultrahigh-spectral-brightness ArF* source in various simple gaseous media. The gases which produced observable signals (H.sub.2, D.sub.2, Ar, Kr, and CO) all have states close to the two-photon energy employed (the CO state involved is actually the same state which enhances the process of the instant invention being approximately equivalent in energy to three of our pump photons), and all are strongly ionized by absorption of the tripled radiation. The use of buffer gases and differentially-pumped chambers is required to minimize this absorption. The method of the instant invention is spared this latter difficulty by the simple fact that the third-harmonic wavelength generated is well below the ionization continuum for CO. The authors of this reference, however, do not mention direct ionization of the non-linear media by the pumping radiation, a problem which can be severe and which solution is taught by our method. This reference, therefore, does not provide any teachings directed toward three-photon harmonic generation in CO with resonant enhancement provided by real energy levels at the energy of the coherently generated photons, nor does it teach one how to overcome the increasingly important (at higher pump energies) ionization of the carbon monoxide gas.
5. The final reference, "Resonantly Enhanced Multiphoton Ionization and Third-Harmonic Generation in Xeonon Gas," by John C. Miller, R. N. Compton, M. G. Payne, and W. W. Garrett, Phys. Rev. Letters, 45, 114 (1980), describes the parallel process of the instant invention for atomic gases. Therein it is noted that the observed multiphoton ionization of the xenon gas nonlinear medium disappears at higher pressures being replaced by intense third-harmonic generation in the forward direction relative to the pump laser. Presumably, at the higher pressures the third-harmonic generation becomes more efficient because of a higher nonlinear susceptibility. No vuv light was detected at right angles to the pump laser either. As mentioned hereinabove in the discussion of Ref. 1, this can be another indication of a strongly phase-matched system. These observations are also found for carbon monoxide in the method of the instant invention. Additionally, we have observed that no multiphoton ionization occurs at any carbon monoxide pressure if the excitation wavelength is adjusted to be to the blue (shorter wavelength) of individual vibronic energy levels of CO. the Miller et al. reference, even when combined with the above-mentioned references, does not teach our invention, however. First, it is not clear that a molecule would stay together (i.e., not ionize or dissociate) at the laser intensities required to generate significant coherent vuv radiation. Remarkably, CO has an about 14 eV ionization potential (significantly higher than the 10 eV known for Xe). This coupled with the resonant enhancement of the nonlinear susceptibility (as in NO) makes CO an ideal candidate to try. Further, it is not obvious that the phase matching in this molecular vapor would be so complete that the ionization process could be virtually eliminated at all pressures by carefully choosing the pump laser wavelength. It is also never possible to make accurate generalizations from atomic properties to molecular ones. Finally, there is no mention of production of coherent, tunable radiation over a wide range of the vuv region in the Miller et al. article. Xenon has only two, three-photon resonant electronic levels in the region of interest (6s(3/2)J= 1 and 6s'(1/2)J=1), and although they can be broadened and shifted at higher pressures, the range coverable is but a fraction of that available from CO with its multitude of vibronic levels. Furthermore, these vibronic levels may be shifted in energy by use of a carbon or oxygen isotope in CO as discussed hereinbelow.
Thus, although coherent third-harmonic generation has been observed in References 2-4 cited hereinabove, the resonantly-enhanced process taught by the method of the instant invention for carbon monoxide with its consequently more modest pump laser requirement, its complete elimination of multiple-photon ionization by judicious choice of pump laser wavelength, and its broad range of tunability which are available only from the choice of a molecular vapor nonlinear medium, is not derivable even for one skilled in the art from these references, singly or in combination.